An internal combustion engine for an uptodated motorcycle is provided with an apparatus which is designed to maintain constant the power output of the engine and accordingly the vehicle speed when desired by the vehicle driver. Such an apparatus serves to save the vehicle driver from fatigue and to lessen the fuel consumption rate of the engine by minimizing changes in the vehicle speed during cruising of the motorcycle. An existing throttle control apparatus of this nature is usually actuated from a manually operated switch and is made inoperative when the switch is reset by the vehicle driver. It is however desirable that the apparatus be disabled without having recourse to the driver's action on the switch. An object of the present invention is to provide an improved throttle control apparatus which can be made inoperative without such a driver's extra action.